Photovoltaic technology offers great potential as an alternative source of electrical energy. That potential has not yet been realized because of the difficulty in making photovoltaic devices that efficiently transform light, such as sunlight, into electricity at a cost that is competitive with conventional energy sources where they are available. Accordingly, researchers are continually striving to improve the efficiency of photovoltaic structures.
With respect to junction photovoltaic devices prepared from semiconductor materials, various approaches have been taken to improve efficiency. For example, photovoltaic structures including several cells connected in series both electrically and optically, with respect to incident light, yield improved efficiency, but at a higher cost of production.
Certain idealized semiconductor structures have been proposed based on particular theoretical energy band configurations that would produce improved efficiency. However, selecting materials that can produce the desired energy band configurations and actually making examples of the idealized structures are problems that have not been solved before. One such theoretical band configuration employs a three layer structure having a wide bandgap energy window layer for passing light without significant loss. The next lower layer, lower with respect to the direction of light incidence, employs a different material having an optical bandgap energy selected to maximize absorption from the spectrum of incident light. A final, still lower or deeper layer, of the same conductivity type as the middle layer, but formed of a different material, acts to aid collection of majority charge carriers, but impede the collection of minority carriers. This heterojunction between the middle and deepest layer is sometimes referred to as a minority carrier mirror since it, in effect, reflects minority carriers
The promise of the idealized three layer structure described has not been previously realized. The number of available semiconductor materials from which to select is limited. The deposition processes for the available materials considered to date to produce the desired energy band configuration usually result in the creation of deleterious interfaces between the different materials.